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Creep in Concrete - Types, Factors Affecting, Effects of Creep

Creep of Concrete

Escalation of stress in cement concrete over time under constant (stable) stress is called creep

In sort deformation of concrete structure under continuous load. under continuous load, For a long duration, the concrete readjust themselves, which behaviour is called creep

The shrinkage and creep occur at the same time and they are assumed to be additive for simplicity

When the sample element is free from load the immediate or elastic recovery is followed by a gradual decrease in strain known as creep recovery

Although creep recovery is much faster compared to creep, the creep strain is not the opposite total

The reversal of creep is not fully complete, and creep is not a simply reversible phenomenon, so that Continuous use of load, even only over a period of a day, results in a residual deformation or permanent deformation or irreversible creep

Creep in Concrete
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If a loaded cement concrete sample is viewed as being subjected to a continuous strain, the creep decreases the stress progressively with time duration. This is known as relaxation

Creep is also called plastic flow, plastic yield or plastic deformation

Creep is a function of time

The creep proportions may be due to the viscous flow of cement paste/slurry due to the closing of the internal voids and the overall crystalline flow

But it is conceded that a large portion is produced by the seepage of colloidal water from the gel formed during the hydration process of cement

The rate of to get rid of the colloidal water is a function of the applied compressive force and of the friction in the capillary channels

If the force is greater, the sleeper the pressure gradient, with a resulting increase in the rate of emitting of moisture and deformation

Types of Creep

Basic creep

Application of constant stress on a concrete specimen under conditions of 100 percent relative humidity, leads to an increase of strain over time, which is called basic creep

Drying creep

The additional creep that occurs when the specimen under load is also drying is called drying creep

Specific creep

Creep strain per unit of the applied stress is called specific creep

Creep coefficient

It is the ratio of ultimate creep strain to elastic strain

Factors affecting creep of concrete

The magnitude of creep depends upon the following factors
  1. Aggregate
  2. Water/cement ratio
  3. Age at the time of loading
  4. Moisture content of the concrete
  5. Humidity of the ambient air
  6. Type of cement
  7. Intensity and duration of stress
  8. Size of the specimen
  9. Temperature

Aggregate

  • Normally, aggregate undergoes very little creep. It is the paste that is responsible for the creep. However, the aggregate provides a restraining effect, when the paste undergoes creep. The stronger the aggregate, the more is the restraining effect and hence the less is the magnitude of creep
  • The modulus of elasticity of aggregate is also one of the important factors influencing creep. The higher the modulus of elasticity of aggregate, the fewer creeps. 
  • Lightweight aggregate shows considerably higher creep than normal-weight aggregate, because of lower modulus of elasticity
  • It has been found that the greater the maximum size of an aggregate, the less is the creep of Concrete

Water/Cement ratio

  • A higher water/cement ratio increases the size of the pores in the paste structure. The amount of paste content and its quality of the mix design is one of the most important factors influencing creep
  • A poor paste structure undergoes higher creep
  • Creep increases with an increase in water/cement ratio
  • All the factors which are affecting the water/cement ratio, also affect the creep of concrete

Age at the time of loading

  • The quality of gel improves with time. Such gel creeps less, whereas a young gel under load being not so stronger creep more. hence, creep decreases with the age of concrete

Age of concrete

Creep

After 1 year

1.0

After 2 year

1.14

After 5 year

1.20

After 10 year

1.26

After 20 year

1.33

After 30 year

1.36

Moisture content of the concrete

  • An increase in the moisture content of concrete, increases creep

Humidity or the ambient air

  • The humidity of the air affects the seepage of moisture from the concrete. An increase in the humidity reduces the rate of loss of moisture and this reduces the seepage. Thus for a given concrete creep is higher
  • The lower the relative humidity. Drying of the concrete while under load enhances creep of concrete

Type of cement

  • The type of cement influences creeps in so far it influences the strength of the concrete at the time of application of the load. Under drying conditions, Portland blast-furnace slag cement results in a higher creep than usual types of Portland cement
  • Finess of cement affects strength development at early ages and thus influences creep. The finer the cement the higher its gypsum requirement, so that re-girding a cement in the laboratory without the addition of gypsum produces improperly regarded cement, which exhibits high creep

Intensity and duration of stress

  • In many tests, a direct proportionality between creep and the applied stress has been found to exist. 
  • Troxelletal found that specimens cured for 90 days and then loaded for 21 years showed 680, 1000 and 14.0 * 10^(-6) creep strains, corresponding to sustained stress levels of 4 MPa, 6 MPa and 8 MPa respectively

Size of the specimen

  • The size of the specimen also affects the magnitude of creep
  • The magnitude of creep decreases with an increase in the Size of the specimen
  • This is due to the reduced seepage as the path travelled by the expelled water is greater with a resulting increase in the frictional resistance to the flow of water from the interior

Temperature

  • The temperature to which concrete is exposed can have two counteracting effects on creep
  • If a concrete member is exposed to a higher than normal temperature as a part of the curing process before it is loaded, the strength will increase and the creep strain would be less than that of a corresponding concrete stored at a lower temperature
  • On the other hand, exposure to high temperature during the period under load can increase creep. The influence of temperature on creep is of considerable interest to nuclear Prestressed Concrete Reactor Vessel (PCRV)

Effects of Creep

  • In reinforced concrete beams, creep increases the deflection with time and maybe a critical consideration in the design
  • In reinforced concrete columns creep results in a gradual transfer of load from the concrete to the reinforcement

  • The extra load in steel is required to be shared by concrete and this situation results in development of full strength of both steel and concrete. This way creep property is useful in reinforced concrete columns
  • However, in eccentrically loaded columns, creep increases the deflection and can lead to buckling

  • In statically indeterminate structures, creep may relieve stress concentrations induced by shrinkage, temperature changes or settlement of supports. In all concrete structures creep reduces internal stresses due to non-uniform load or restrained shrinkage

  • In mass concrete, creep in itself may be a cause of cracking when restrained concrete mass undergoes a cycle of temperature change due to the development of the heat of hydration and subsequent cooling

  • Therefore, all precautions and steps must be taken to see that increase in temperature does not take place in the interior of mass concrete structures like concrete dams, retaining wall etc.
  • The rise in the temperature in the interior of a large concrete mass must be controlled by the use of low heat cement, a low cement content, precooling of mix design ingredients, limiting the height of concrete lifts, and cooling of concrete by circulating refrigerated water through a network of pipes embedded in the concrete mass

  • In the case of prestressed concrete, creep reduces prestress and provision is the mage for the loss of prestress in the design of such structures

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3 comments

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